In recent years, to remove the nitrogen oxides (NOx) contained in the exhaust gas of an internal combustion engine, it has been proposed to provide an NOx selective reduction catalyst for selectively reducing the NOx in the exhaust gas inside an exhaust passage of the internal combustion engine. The NOx selective reduction catalyst adsorbs ammonia and makes the NOx in inflowing exhaust gas react with the ammonia to reduce and remove the NOx. For example, in such an exhaust purification system, it would be desirable if ammonia and NOx could be detected.
For this reason, a multiple gas detection device able to detect the concentration of NOx and the concentration of ammonia contained in measured gas has been proposed (for example, PLT 1). In particular, the multiple gas detection device described in PLT 1 is provided with an NOx sensor part detecting the concentration of NOx contained in measured gas and two ammonia sensor parts detecting the concentration of ammonia contained in the measured gas. These two ammonia sensor parts are configured so that the ratios of sensitivity to ammonia and the sensitivity to NOx differ from each other. According to PLT 1, it is considered that such a configured multiple gas detection device can detect the concentration of NOx and the concentration of ammonia contained in measured gas.